1. Heat the oil in a skillet over medium heat. Add the onions and cook, stirring occasionally, until deeply caramelized, around 25 to 30 minutes.
2. In a large mixing bowl, combine the beef, eggs, breadcrumbs, milk, salt, pepper, and Worcestershire sauce. Mix well and then transfer the mixture to a piece of waxed paper or parchment paper. Pat the mixture out into a large rectangle. Sprinkle the cheese over the top, leaving a border of about 1/2 inch around the edge.
3. Starting with one of the short sides of the rectangle, roll the meat up around the cheese, jelly roll style. Press the ends together to seal the filling inside. Transfer to the slow cooker.
4. Cover and cook on low for 8 hours or high for 4 hours.
Let it rest for about 5 minutes before slicing and serving.
